Line Strength

[DDW]

You are all confusing the effort required to lift a weight with the breaking strength of the line, in this case we are assuming 5000# breaking strength. Forgetting for a moment that lines are rated lower than their actual failure point, you could lift 5000#. The purpose of the ropes and pulleys (on a boat, that is a "purchase") is to lower the amount of work that is required to be expended to lift that weight, by a factor of the number of pulleys and the number of "lines," making it possible for one person to lift a much larger load than they could directly. The earlier poster was correct, the breaking strength of the rope must be less than the load or you will drop it.

The purpose of a tackle is also to be able to lift something heavier than the line's load rating or breaking strength. A 5000 lb strength line on a 4:1 tackle will fail at 20,000 lbs load on the tackle, not 5000, if the blocks are rated for 20,0000. If the safe load for the 5000 lb line is 2000 lbs, then a 4:1 tackle using it will lift 8000 lbs safely (provided the blocks are up to it) - much higher than the 5000 lb breaking load of the line.

 

[Solly]

Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.

 

[diver dave]

Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.

But if the block is below the array of lines, the story changes.

 

[diver dave]

A basic statics course will show all line segments see the same force when only threaded around free pulleys

 

[Bacchus]

Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.

But the force on the line does change - If you only apply 2500 # to lift it that's all the force there is on the line at any point

Solly
Read and study xsbank's link - the weight of the item being lifted is NOT the load on the line (or force req'd to lift the wt) when a multi line / pulley system is used - That's the magic and physics of the basic design... just ask the Eqyptians who built the pyramids... mechanical advantage is a wonderful thing.

The other way to find the advantage by empirical means vs theoretical is to pull in a length of line and measure the distance the load is lifted
- if you pull 2X as much line as the load is lifted the force on the line is 1/2 of the wt of the load - 2:1 mech advantage
- If you need to pull in 4X the distance the load is lifted the mech adv is 4:1 and the force req'd is 1/4 the wt of the load.

That's just the way it is

 

[Delfin]

Pretty sure the mechanical advantage imparted by using pulleys doesn't change the tensile strength of the line. That is determined by the type, braid, etc of the actual line itself.

Yep. If that weren't the case, oil tankers could tie up to the wharf with 1/4" clothes line and a bunch of 1/4" pulleys.

 

[Delfin]

But the force on the line does change - If you only apply 2500 # to lift it that's all the force there is on the line at any point

Solly
Read and study xsbank's link - the weight of the item being lifted is NOT the load on the line (or force req'd to lift the wt) when a multi line / pulley system is used - That's the magic and physics of the basic design... just ask the Eqyptians who built the pyramids... mechanical advantage is a wonderful thing.

The other way to find the advantage by empirical means vs theoretical is to pull in a length of line and measure the distance the load is lifted
- if you pull 2X as much line as the load is lifted the force on the line is 1/2 of the wt of the load - 2:1 mech advantage
- If you need to pull in 4X the distance the load is lifted the mech adv is 4:1 and the force req'd is 1/4 the wt of the load.

That's just the way it is

And the "way it is" with any lifting tackle is that you have a single lead of line either on the hoist side or both sides. And that single lead doesn't gain magical powers just because the line is doubled, tripled or whatever elsewhere.

 

[Bacchus]

Pretty sure the mechanical advantage imparted by using pulleys doesn't change the tensile strength of the line. That is determined by the type, braid, etc of the actual line itself.

Yep. If that weren't the case, oil tankers could tie up to the wharf with 1/4" clothes line and a bunch of 1/4" pulleys.

You are absolutely correct pulleys don't/can't change the strength or any other property of the line.
What pulleys / blocks do is reduce the load on the line so multiple weaker lines are capable of lifting a heavier load that would break a single line of the same size / strength.
In theory the tanker example would work if enough smaller lines were employed AND the pulleys & their attachment were capable of handling the loads.
It seems like folks are reluctant to read and understand the link xsbank provided or think it's a hoax???

 

[Bacchus]

And the "way it is" with any lifting tackle is that you have a single lead of line either on the hoist side or both sides. And that single lead doesn't gain magical powers just because the line is doubled, tripled or whatever elsewhere.

That's true with one single pulley but not with multiple pulleys / blocks.
Have you looked at the link?
Don't understand it or just don't believe it?
You don't have to take my word for it.

Why would companies ever be able to sell blocks/tackle if they don't provide a mechanical advantage?
Have you ever used a block & tackle, chain fall or come along to lift or pull something?

 

[Delfin]

That's true with one single pulley but not with multiple pulleys / blocks.
Have you looked at the link?
Don't understand it or just don't believe it?
You don't have to take my word for it.

Why would companies ever be able to sell blocks/tackle if they don't provide a mechanical advantage?
Have you ever used a block & tackle, chain fall or come along to lift or pull something?

The question was whether a tackle increased the tensile strength of the line, so I have a simple question for you Bacchus. If you have a lifting tackle, is there a single line that you pull on or that attaches to the thing you are lifting somewhere in the tackle? Pro tip: If there isn't, you don't have a tackle, you have a continuous line that is doubled, tripled, etc. around a bunch of pulleys.

Stick as many pulleys into a tackle as you like. You won't change the tensile strength of the line, although you will reduce the load on the line when in use. Assuming, as you seem to, that the tensile strength does change, then if a car has enough horsepower to get to 100 mph, dropping it off a cliff increases the horsepower.

Perhaps you didn't understand the OP's question, or prefer to answer one he didn't ask?

 

[Delfin]

If the line is 1000# tensile strength and the weight is 1500#, Superman here will put 750# of force on the line to lift the weight. If the weight is 2001#, then the 1000# tensile strength line will break because Superman will have to exert 1000# 8oz of force to lift the weight. If you can demonstrate that this is incorrect, then a perpetual motion machine is within your reach.

 

[Delfin]

Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.

What's amazing is that this question has created 3 pages of responses. To be charitable, we should assume that those still scratching their heads over this conundrum are, as XS said, confusing tensile strength of the line in a tackle with the mechanical advantage the tackle bestows.

 

[DDW]

Unfortunately, the original question was asked in a confused way. Of course a tackle does not change the tensile strength of a line, but the tackle will change the amount of weight that the line can lift, provided the tackle hardware is up to it. Read carefully, the OP asked if the 4 parts of the tackle increased the tensile strength of the tackle, not the line:

According to one strength chart I checked, the tensile strength of this line is 5,000 lbs. There are 5 pulleys that control one pole, thus 4 loops from the crane (?) to the near top and mid point of the pole. Does this translate to 20,000 lbs tensile strength?

The answer is "yes".

So yes, you can tie an oil tanker to the wharf with 1/4" clothesline, theoretically, though it would be practically inconvenient. On large very highly loaded sailing rigs, lashings are increasingly used in standing rigging. These make use of multiple turns of light line whose tensile strength is well below the working load of the rigging, yet multiple turns of this line well exceed the working load.

 

[Delfin]

Unfortunately, the original question was asked in a confused way. Of course a tackle does not change the tensile strength of a line, but the tackle will change the amount of weight that the line can lift, provided the tackle hardware is up to it. Read carefully, the OP asked if the 4 parts of the tackle increased the tensile strength of the tackle, not the line:



The answer is "yes".

So yes, you can tie an oil tanker to the wharf with 1/4" clothesline, theoretically, though it would be practically inconvenient. On large very highly loaded sailing rigs, lashings are increasingly used in standing rigging. These make use of multiple turns of light line whose tensile strength is well below the working load of the rigging, yet multiple turns of this line well exceed the working load.

Setting aside that block and tackles don't have tensile strength but mechanical advantage based on the number of wraps, the answer is still "no". Tensile strength is the amount of force that can be applied before the line breaks and that doesn't change regardless of how many pulleys there are in a block and tackle. In the OP's situation, the line will still break if he applies more than 5000# of force, even if he is able to lift 5000# with only 1250# of force. Attach that arrangement to a weight of 20,000# and the line will break.

And no, you can't tie up an oil tanker with 1/4" clothes lines by wrapping it around a bunch of pulleys, unless you eliminate the hauling part of the block and tackle. If you do, you no longer have a block and tackle as the OP describes, but a set of loops of line that will have a tensile strength equal to the number of loops times the tensile strength of one line. A totally different proposition from the arrangement for passive fins. There, a block and tackle is employed to reduce the amount of effort you need to pull the dolphin out of the water. That mechanical advantage has no impact on the tensile strength of the line.

 

[Bacchus]

Last post and I'm done
Nobody here claimed anything... pulley, block or a magic hat can change the properties of any line utilized so we are in agreement there.
What several have tried to point out is that a line can support or lift many times its breaking strength by employing a block (multiple pulleys and wraps).
I'm not sure if you understand or are arguing that point... but I'm done.
I do hope the OP got his question answered.

Take the pulley Test

 

[diver dave]

Setting aside that block and tackles don't have tensile strength but mechanical advantage based on the number of wraps, the answer is still "no".

This seems to be the sticking point in the argument. Why can't a block and tackle have a tensile strength?

 

[DDW]

Setting aside that block and tackles don't have tensile strength .....

And no, you can't tie up an oil tanker with 1/4" clothes lines by wrapping it around a bunch of pulleys, unless you eliminate the hauling part of the block and tackle. If you do, you no longer have a block and tackle as the OP describes, but a set of loops of line that will have a tensile strength equal to the number of loops times the tensile strength of one line.

Well we are just arguing semantics here but:

Everything has a tensile strength, including a tackle. Tensile strength is simply the load that can be applied to each end of an object without tensile failure.

A block and tackle with the running end tied off, is just a set of loops of line. And it can be used to tie up a tanker. It doesn't matter if the running end is tied off, or being held still by someone, it's still some loops of line and behaves exactly the same.

If you define a tackle narrowly as an arrangement of blocks and line in motion, that becomes a set of loops (and no longer a tackle) as soon as the running end is held still and motion stops, then what you say is true - but most would not agree with that definition. A tackle is still a tackle, even when tied off.

 

[psneeld]

Wow, anyone with a better link or real qualifications want to weigh in here?

I have my opinion, but there are some smart people here who have me doubting my many years of practical experience of dealing with blocks and tackle.

 

[Delfin]

This seems to be the sticking point in the argument. Why can't a block and tackle have a tensile strength?

The line certainly has tensile strength. And if a block and tackle has a tensile strength it would only be because the line has one. The question was whether that strength of the line changes in a block and tackle and the answer to that is of course not, even if you can lift with less effort.

I think the bigger area of confusion is that some aren't considering that for any arrangement to be a "block and tackle", there is a hoist line, which is a single strand of whatever you're using. The tensile strength of that is the tensile strength of the whole system. For the OP, that is 5,000#, the same it would be without a block and tackle. Further, even if the load on the line in lifting a given weight decreases in a block and tackle, the tensile strength of the line certainly doesn't change. Now, if you do away with the live hoist end, and just double up the line, then there is an increase in tensile strength, but then you don't have a block and tackle, which is what the OP describes and what his question pertained to.

 

[Delfin]

Well we are just arguing semantics here but:

Everything has a tensile strength, including a tackle. Tensile strength is simply the load that can be applied to each end of an object without tensile failure.

A block and tackle with the running end tied off, is just a set of loops of line. And it can be used to tie up a tanker. It doesn't matter if the running end is tied off, or being held still by someone, it's still some loops of line and behaves exactly the same.

If you define a tackle narrowly as an arrangement of blocks and line in motion, that becomes a set of loops (and no longer a tackle) as soon as the running end is held still and motion stops, then what you say is true - but most would not agree with that definition. A tackle is still a tackle, even when tied off.

Just to make sure we aren't talking past each other, it sounds like what you are saying is that if I have a 2 wrap block and tackle with a 1,000# tensile strength line, that tackle now has a tensile strength of 2000# and can lift 2000# without breaking as long as the hoist is tied off, say, to a cleat, or held by someone?

 

[koliver]

The fallacy in most of this thread is to suggest exceeding the breaking strength of the line used. For argument, give yourself a line that has enough strength so that even if used in a 4 part tackle, your fall has the strength needed to lift the load in a single pull. Then you only need to count the parts that give you mechanical advantage to know the total weight the tackle can lift. At the end of the fall, however, you are using a single part, so that one section needs to be strong enough for the full weight.
For example, if you want to lift 2000 lbs and use a 4 part tackle, you need to use a line with a breaking strength of more than 2000 lbs, but your winch can be turned with the same force as that needed to lift 500 lbs on a single part line, it just has to turn 4 times as many turns, thus lifting 2000 lbs.
The line will still register the full 2000 lb weight.

 

[diver dave]

For example, if you want to lift 2000 lbs and use a 4 part tackle, you need to use a line with a breaking strength of more than 2000 lbs, but your winch can be turned with the same force as that needed to lift 500 lbs on a single part line, it just has to turn 4 times as many turns, thus lifting 2000 lbs.
The line will still register the full 2000 lb weight.

True for a winch with a reduction gear, definitely not true for a block and tackle. If you have 4 lines headed for the bottom block, the load is shared by all. And, in every line segment, including the free end, the tension is the same and is 1/4 of the load supported.

 

[diver dave]

https://www.slideshare.net/superegofrued/statics-free-body-diagram

this will save you from attending the first 3 weeks of statics class.

 

[psneeld]

Funny, I keep reading technical explanations that say that the line does NOT have to have the safe working load as the weight being lifted, but actually some reduced value involving other calculations other than mechanical advantage ones.

My suggestion is go to some "rigging" engineering websites if anyone is still undecided.

 

[DNT99611]

If the line is 1000# tensile strength and the weight is 1500#, Superman here will put 750# of force on the line to lift the weight. If the weight is 2001#, then the 1000# tensile strength line will break because Superman will have to exert 1000# 8oz of force to lift the weight. If you can demonstrate that this is incorrect, then a perpetual motion machine is within your reach.

Just for fun. The hook above the load is seeing 1500 pounds, the load lifted. The attachment point above the upper pulley is seeing 2250 pounds of force.

 

[diver dave]

Depends on the pull angle. At about 45 deg roof pull is less than 2200. Neglecting weight of pulleys and line, just under 2100 lbs. If the pull is made straight down, then it is 2250. Straight to the side, then the roof attach will see a shear load of 750 and a down force of 1500.

 

[DDW]

Just to make sure we aren't talking past each other, it sounds like what you are saying is that if I have a 2 wrap block and tackle with a 1,000# tensile strength line, that tackle now has a tensile strength of 2000# and can lift 2000# without breaking as long as the hoist is tied off, say, to a cleat, or held by someone?

Not sure what you mean by a "2 wrap" tackle, but I'll assume a 2 part or 2:1:

The tackle has a tensile strength of 2000 lbs in your example, whether the running end is tied off or held or moving. Can it lift safely 2000 lbs? well, no, that is it's breaking point. It can lift 1999 lbs, and perhaps 500 lbs safely (with a 4:1 margin).

Let's be clear: a tackle has a running line rove through it, blocks at each end (and perhaps in the middle in a compound tackle), and additional attachments at each end fixing it to the anchoring point and load, these additional attachments might also be lines, but they are not the running lines of the tackle and not what we are talking about. Each of these has a tensile strength when considered individually. Let us assume that the blocks and additional attachments are strong enough that the running line will be the first thing to break.

In a simple 4:1 tackle, there are four sections of line that get shorter when I pull the running end. Any load applied to the tackle blocks will be shared by all 4 parts, and the tension in the line will be 1/4 of that load. A 100 lb pull applied to that line will put 100 lbs of tension into the line, in all parts of it, and 400 lbs pull on the each block. A line of 101 lbs breaking strength is sufficient to do this, with no margin for safety. You do not need a 400 lb strength line, the line only sees 100 lbs tension, anywhere in it.

If there is an additional, separate line involved attaching the block to the load or fixed end, then it must have a breaking strength of higher than 400 lbs. But that is a separate piece of line, not part of the running line of the tackle.

I am an engineer, I do these calculations all the time. I also sell commercially a piece of sail handling gear that depends on them, to the world's finest sailboats. This isn't rocket science, but it IS science and there is a right answer.

 

[Bacchus]

Not sure what you mean by a "2 wrap" tackle, but I'll assume a 2 part or 2:1:

The tackle has a tensile strength of 2000 lbs in your example, whether the running end is tied off or held or moving. Can it lift safely 2000 lbs? well, no, that is it's breaking point. It can lift 1999 lbs, and perhaps 500 lbs safely (with a 4:1 margin).

Let's be clear: a tackle has a running line rove through it, blocks at each end (and perhaps in the middle in a compound tackle), and additional attachments at each end fixing it to the anchoring point and load, these additional attachments might also be lines, but they are not the running lines of the tackle and not what we are talking about. Each of these has a tensile strength when considered individually. Let us assume that the blocks and additional attachments are strong enough that the running line will be the first thing to break.

In a simple 4:1 tackle, there are four sections of line that get shorter when I pull the running end. Any load applied to the tackle blocks will be shared by all 4 parts, and the tension in the line will be 1/4 of that load. A 100 lb pull applied to that line will put 100 lbs of tension into the line, in all parts of it, and 400 lbs pull on the each block. A line of 101 lbs breaking strength is sufficient to do this, with no margin for safety. You do not need a 400 lb strength line, the line only sees 100 lbs tension, anywhere in it.

If there is an additional, separate line involved attaching the block to the load or fixed end, then it must have a breaking strength of higher than 400 lbs. But that is a separate piece of line, not part of the running line of the tackle.

I am an engineer, I do these calculations all the time. I also sell commercially a piece of sail handling gear that depends on them, to the world's finest sailboats. This isn't rocket science, but it IS science and there is a right answer.

...

 

[AusCan]

Whew! I'm glad that got cleared up. Physics 101 can get overcomplicated sometimes.

 

[Rain Dog]

I just can't quit this thread. I hope everyone ends up on the same (correct) page in the end! Reminds me of the Monty Hall Problem.